A customs union implies the unification of two or more sovereign countries into one customs area with a common customs tariff and a customs tariff. In this graduate thesis, the concept of the customs union will be elaborated on the general level, its definition and the same will be elaborated in detail. Following the foregoing, the objectives and assumptions of the creation or qualification of the customs union will be explained. Also, the differences between the Customs union and the Free Trade Agreement will be elaborated in detail. As a special chapter, the history and the emergence of customs unions will be elaborated in order to understand more closely what they have derived from. Following this, the current members of the Customs union of the European union Customs union will be announced, with a special focus on the legal framework of the European Union customs union, which is further elaborated below. The mentioned context will also mention the Republic of Croatia as one of the members of the Customs union of the European union and the question of the application of European customs regulations. In keeping with the foregoing, the legal arrangement of the customs union in the European union and the way in which this legal framework will be created will be elaborated and will further explain the importance of the customs tariff. At the end of the paper, a concluding review of the thematic problem is given.